Compact fiber optic geometry for a counter chirp fmcw coherent laser radar

ABSTRACT

A system and method for determining a measured distance between a measuring device ( 20 A) and an object ( 21 ), the system including a first laser source ( 13 ) for producing a first light beam ( 13 A) having a first waveform ( 32 ) and a first frequency; a second laser source ( 11 ) for producing a second light beam ( 1  IA) having a second frequency, said second light beam (HA) having a second waveform ( 36 ), wherein said first frequency is chirped up at the first rate as said second frequency is chirped down at the first rate, and said second frequency is chirped up at the second rate as said first frequency is chirped down at the first rate; an optical element ( 15 ) for combining said first light beam ( 13 A) with said second light beam (HA) into a combined light beam path ( 17 ), said optical element ( 15 ) splitting a returning portion of said combined light beam path ( 17 ) into a third light beam ( 24 ); and a single detector ( 23 ) for receiving said third light beam ( 24 ) including two different beat frequencies that are proportional to the measured distance.

BACKGROUND

The present embodiment relates generally to optical sensors formeasuring distances to objects (targets).

While it is known to transfer light through optical fibers, precisioncan be compromised due to the environmental effects on the fiber itself.These environmental effects can change the optical path length and thepolarization of the light in the fiber, and can adversely affectmeasurement precision. The use of optical heterodyne detection can allowfor optical radiation detection at the quantum noise level. As such,coherent optical systems can provide improvements in range, accuracy,reliability, scanning range, working depth of field, and operation inambient light conditions. Furthermore, a coherent system can obtainsufficient information about the characteristics of a target locationquickly.

Optical heterodyne detection includes a source light beam which isdirected to a target and reflected therefrom. The return light beam isthen mixed with a local oscillator light beam on a photo detector toprovide optical interference patterns which may be processed to providedetailed information about the target. Optical heterodyne techniques cantake advantage of the source and reflected light beam reciprocity. Forexample, these light beams can be substantially the same wavelength andare directed over the same optical axis. In this case, thesignal-to-noise ratio (SNR) is sufficiently high so that a smallreceiving aperture may be used, for example, a very small lens capableof being inserted into limited access areas. Since a small receiveraperture can provide detailed information about the target, the opticalcomponents of a coherent system may be made very small.

Precision FM laser radars can incorporate a single chirp laser sourceand a polarization maintaining fiber optic geometry with separate localoscillator (LO) and signal paths. What is needed is a counter-chirpconfiguration that is made insensitive to vibration induced range errorsby an accurate Doppler correction. What is further needed is combiningthe LO and signal paths for two lasers into a single fiber, so that thefiber optic circuit is less complicated, less expensive due to fewercomponents, and immune to error caused by changes in the LO and signalpath lengths that result from environmental factors such as temperaturechanges. For example, the manufacturing industry, in which bothbackground vibrations and changing environmental conditions exist, couldbe a candidate user for this laser configuration. The combination of LOand signal paths can provide the additional benefit that the sensor headportion of the unit can be placed in areas of restricted volume since itcan be remoted arbitrarily far from the rest of the unit.

In summary, what is needed is a practical optical precision measurementsystem capable of great accuracy, rapid measurement time, access totight spaces, flexibility, and reliability.

SUMMARY

The needs set forth above as well as further and other needs andadvantages are addressed by the embodiments set forth below.

The present embodiment relates to an optical distance measuringapparatus that can include, but is not limited to including, a firstlaser source for producing a first light beam, a second laser source forproducing a second light beam, where waveforms for the first and secondlight beams are 180 degrees out of phase with each other so that thefirst light beam is chirped up as the second light beam is chirped downand vice versa, a first optical element for combining the first andsecond light beams into a combined light beam, and for splitting anyreturning portion of the combined light beam into third light beam, anda first detector for receiving the third light beam, In anotherembodiment, a method for determining a range of a distant object isenvisaged, including producing a first light beam from a first lasersource and a second light beam from a second source, where waveforms forthe first and second light beams are 180 degrees out of phase with eachother so that the first light beam is chirped up as the second lightbeam is chirped down and vice versa, directing light emitted from thesources toward an object, receiving a reflected LO path andtarget-reflected signal path for each source, detecting the LO path andsignal path for each source, and heterodyning the paths for each sourceto generate a beat frequency, which frequency is proportional to therange difference between the two paths, and where the path lengthdifference between the LO path and corresponding signal path is equal tothe distance to be measured.

The method for determining a measured distance between an output of ameasuring device and an object of the present embodiment can include,but is not limited to including, the steps of producing a first lightbeam having a first frequency from a first laser source and a secondlight beam having a second frequency from a second laser source,chirping up the first frequency at a first rate as the second frequencyis chirped down at a second rate, chirping up the second frequency atthe second rate as the first frequency is chirped down at the firstrate, combining the first light beam and the second light beam,directing the combined light beam path toward the object, receiving areflected local oscillator (LO) path light beam associated with thecombined light beam path, receiving a target-reflected signal path lightbeam associated with the combined light beam path, and heterodyning theLO path light beam and the target-reflected signal path light beam togenerate two different beat frequencies that are proportional to themeasured distance, the two beat frequencies being detected by a singledetector.

The system for determining a measured distance between a measuringdevice and an object can include, but is not limited to including, afirst laser source for producing a first light beam having a firstwaveform and a first frequency, a second laser source for producing asecond light beam having a second frequency, the second light beamhaving a second waveform, wherein the first frequency is chirped up at afirst rate as the second frequency is chirped down at a second rate, andthe second frequency is chirped up at the second rate as the firstfrequency is chirped down at the first rate, an optical element forcombining the first light beam with the second light beam into acombined light beam path, the optical element splitting a returningportion of the combined light beam path into a third light beam, and asingle detector for receiving the third light beam including twodifferent beat frequencies that are proportional to the measureddistance.

In somewhat less general terms the present embodiment is a diplexdual-chirp laser apparatus for precision absolute distance measurementcomprising a first frequency-modulated laser that emits a first coherentlight beam having a first emission frequency modulated by a firstchirping modulation signal and a second frequency-modulated laser thatemits a second coherent light beam having a second emission frequencymodulated by a second chirping modulation signal. The second chirpingmodulation signal is established to chirp with a constant phasedifference from the first chirping modulation signal, preferably a 180°phase difference. Furthermore, the chirp rate of the second laser issufficiently different from that of the first that two distinctlymeasureable beat frequencies are produced at the detector. The presentembodiment can further comprise a fiber optic coupler opticallyconnected to the first frequency-modulated laser wherein the firstcoherent light beam is split into two nominally 50% fractions.Additionally the fiber optic coupler is optically connected to thesecond frequency-modulated laser, wherein the second coherent light beamis split into two nominally 50% fractions.

There is also an interface at the beam exit end of the linearpolarization-maintaining fiber, whereby a fraction of the transmittedlight from each of the first coherent light beam and the second coherentlight beam is reflected back toward the fiber optic coupler and thencetransmitted into the photodetector and whereby a complementary fractionof the transmitted light is transmitted to the target and returned tothe interface from the target; an optical detector optically connectedto the fiber optic coupler; whereby a first interference is establishedbetween the reflected fraction of the transmitted light from the firstcoherent light beam and the return of the first coherent light beam fromthe target and whereby the optical detector detects a first beatfrequency from the first interference and a second beat frequency fromthe second interference whereby a second interference is establishedbetween the reflected fraction of the transmitted light from the secondcoherent light beam and the return of the second coherent light beam.Thereafter the beat frequency signals are sent to digital signalprocessing apparatus configured to produce a first beat frequencymeasurement and a second beat frequency measurement. In the presentembodiment a calibrated reference arm standard can be opticallyconnected to the fiber optic coupler and can receive one of the twonominal 50% fractions of the first coherent light beam and furtherproducing a first reference arm output; this reference arm also receivesone of the two nominal 50% fractions of the second coherent light beamand further producing a second output frequency. Computer apparatus cancombine the first beat frequency measurement with the first referencearm output to produce a first absolute distance measurement, and furthercan combine the second beat frequency measurement with the secondreference arm output to produce a second absolute distance measurement.The computer apparatus can further combine the first absolute distancemeasurement with the second absolute distance measurement to produce acomposite absolute distance measurement, wherein uncertainty in thecomposite absolute distance measurement is substantially reduced.

Optionally the calibrated reference arm standard is a fiber opticinterferometer. Optionally as well, the fiber optic interferometer caninclude a first fiber optic coupler optically connected to a secondfiber optic coupler, the first fiber optic coupler capable of splittingincoming light into a two parts; two fiber optic fibers of differentoptical path lengths having a calibrated optical path length differenceand each receiving a part of the incoming light from the first fiberoptic coupler; coupler termination at the end of each fiber whereby thelight is reflected back to the fiber coupler, whereby the two parts ofthe light from the two fiber optic fibers are recombined, whereby aninterference and a consequent beat frequency are established; a detectorthat detects the beat frequency. The results proceed to digital signalprocessing apparatus configured to produce a beat frequency measurementfor each laser, and said beat frequency measurement for each lasercomprises the reference arm output for the reference arm standard.

The present embodiment can be applied in non-contact precision distancemeasuring applications in unstable environments such as exist inairplane and automotive factories, for example, for in-line measurementsof parts as they are being assembled.

For a better understanding of the present embodiments, together withother and further embodiments thereof, reference is made to theaccompanying drawings and detailed description.

DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a graphical diagram of the laser optical frequency and theheterodyned radio frequency (RF) signal of coherent laser radar;

FIG. 2 is a graphical diagram of the linear frequency modulation, or“chirp,” together with the corresponding “beat” frequency that resultsfrom combining the LO and target light signals for first laser source 13and second laser source 11 in the presence of relative target motion;

FIG. 3 is a schematic diagram of a configuration having two lasersources in a counter chirp configuration;

FIG. 4 is a schematic diagram of an alternate embodiment that canminimize back reflections and also make the circuit easier to implementon planer lightwave circuits;

FIG. 5A is a schematic diagram of a further alternate embodiment havinga 3×3 polarization maintaining coupler, where the embodiment couldincrease the sensitivity of the system by decreasing the amount of noisegenerated by light backscattering along the output fiber;

FIG. 5B is a schematic diagram of a further alternate embodiment havinga 2×2 polarization maintaining coupler, where the embodiment could alsoincrease the sensitivity of the system by decreasing the amount of noisegenerated by light backscattering along the output fiber;

FIG. 6 is a schematic diagram of a further alternate embodiment havingmultiple output beams;

FIG. 7 is a schematic diagram of a geometry that uses delay lines tomultiplex the various output signals onto one detector, thus reducingthe cost and complexity of the radar;

FIG. 8 is a schematic diagram of an alternate embodiment a configurationof the multi-beam concept;

FIG. 9 is a schematic diagram of another alternate multi-outputembodiment;

FIG. 10 is a schematic diagram of coupling the light from a fiberpigtailed visible laser diode into an output fiber; and

FIG. 11 is a schematic diagram of the generation of local oscillator andsignal paths.

DETAILED DESCRIPTION

Before the present embodiments are described, it is understood that thisdisclosure is not limited to the particular devices, methodology andcomponents described as these may vary. It is also to be understood thatthe terminology used herein is for the purpose of describing particularembodiments only, and is not intended to limit the scope of thisdisclosure. The following configuration description is presented forillustrative purposes only. Any configuration and architecturesatisfying the requirements herein described may be suitable forimplementing the system and method of the present embodiments.

It should be further understood that as used herein and in theindependent claims, the singular forms “a,” “an,” and “the” includeplural reference unless the context clearly dictates otherwise. Thus forexample, reference to “an isolator” includes a plurality of suchisolators, reference to a “lensing system” is a reference to one or morelenses and equivalents thereof known to those skilled in the art. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meanings as commonly understood by one of ordinary skill in theart.

Referring to FIG. 1, in coherent or FM laser radar that uses a diodelaser as its source, the frequency of the laser is modulated directly bymodulating the laser's injection current. Typically, the frequency ismodulated with a waveform with the objective of producing linearmodulation. This type of modulation is often referred to as a chirp. Thetwo chirping modulation input signals or waveforms that are sent to thetwo lasers to modulate their output wavelength are not identical. Eachlaser is unique in how it tunes and therefore a unique waveform must begenerated for each laser. Both the waveform shape and amplitude varyfrom laser to laser. It is important to generate an input injectioncurrent waveform that produces a linear chirp of a given change inwavelength over the duration of the chirp. In a common form ofmodulation for this type of application, the injector current modulationsignals are uniquely shaped for each laser and are distorted sawtoothwaves intended to produce a linear sawtooth frequency modulationenvelope for the output of the laser. The basis of coherent FMCWreflectometry is the heterodyne mixing of two signals originating fromthe same linearly chirped source, one signal following a localoscillator “LO” path, while the other reflects back from the target. Anytime delays between the signals reflected back from sites along the testpath and the signal from the reference reflection give rise to beatfrequencies in the mixed output. The values of the beat frequencies areproportional to the time delays, while the sizes of the signals at thebeat frequencies are proportional to the corresponding reflectionfactors. A spectral analysis of this output therefore reveals thelocations (relative to the reference path length) and strengths of anysites of reflection along the test path.

Referring now to FIG. 1, the frequencies of LO light and signal lightfor each of first laser source 13 and second laser source 11 aregraphically depicted as waveforms 32, 34, 36, and 38. For the case of norelative motion between the radar and target 21 (used interchangeablyherein with object 21), there is no Doppler shift and the beat frequencygenerated by mixing the LO and signal light is the same for the upsweepand the down sweep of each of first laser source 13 and second lasersource 11. For first laser source 13, beat frequency is f₁ 33, andsecond laser source 11, beat frequency is f₂ 35. Modulation period 42,T_(mod), is identical for both first laser source 13 and second lasersource 11, and first laser source 13 is 180° out of phase with secondlaser source 11. The upsweep of first laser source 13 occurs during thedownsweep of second laser source 11. Both lasers are swept about thesame nominal frequency but first laser source 13 is swept a greateramount, Δf₁ 44, than laser two, Δf₂ 46. In this case the ratio of f₁ 33to f₂ 35 is the same as the ratio of Δf₁ 44 to Δf₂ 46. By tuning firstlaser source 13 and second laser source 11 at two different rates, asingle detector 23 can be used to detect the two mixed signals reducingthe number of components needed. The two signals are at separatefrequencies and can each be measured using digital signal processingtechniques.

Referring now to FIG. 2, the linear frequency modulation, or “chirp,”together with the corresponding “beat” frequency that results fromcombining the LO and target light signals for first laser source 13 andsecond laser source 11 in the presence of relative target motion aregraphically depicted. In exemplary FIG. 2, the laser base frequency isapproximately 200 terahertz, and the “beat” frequency is in the 1 MHzrange. If the surface being measured is moving relative to first lasersource 13, the beat frequencies corresponding to laser upsweeps will bedifferent from the beat frequencies corresponding to the downsweeps, dueto Doppler frequency shifting. Measuring the frequency differencebetween signals can enable a determination of velocity to be made. Ifthe range and velocity of the target are constant during one modulationperiod, f_(1up) and f_(1down) can be expressed as

f _(1up) =|f ₁ −f _(d)|  (1)

f _(1down) =|f ₁ +f _(d)|  (2)

where f₁ is the frequency due to range and f_(d) is the frequency due tothe Doppler shift. If f₁>f_(d) then

$\begin{matrix}{f_{1} = \frac{f_{1\; {up}} + f_{1\; {down}}}{2}} & (3) \\{f_{d} = \frac{f_{1\; {up}} - f_{1\; {down}}}{2}} & (4)\end{matrix}$

and range and velocity can be expressed as

$\begin{matrix}{f_{1} = {\frac{f_{1\; {up}} + f_{1\; {down}}}{2} = \frac{2\; R\; \Delta \; f_{1}}{{cT}_{mod}}}} & (5) \\{f_{d} = {\frac{f_{1\; {up}} - f_{1\; {down}}}{2} = \frac{2\; v}{\lambda}}} & (6)\end{matrix}$

where λ is the optical wavelength and ν is the relative velocity.

Similarly,

$\begin{matrix}{f_{2} = {\frac{f_{2\; {up}} + f_{2\; {down}}}{2} = \frac{2\; R\; \Delta \; f_{2}}{{cT}_{mod}}}} & (7) \\{f_{d} = {\frac{f_{2\; {down}} - f_{2\; {up}}}{2} = \frac{2\; v}{\lambda}}} & (8)\end{matrix}$

Continuing to refer to FIG. 2, if the surface being measured is movingrelative to the laser light source, the beat frequencies correspondingto laser upsweeps will be different from the beat frequenciescorresponding to the downsweeps, due to Doppler frequency shifting.Measuring the frequency difference between these signals can enable adetermination of velocity to be made. While processing up and downchirps from a single laser allows for detecting separately the range andvelocity of the target, the range data may be compromised if there isany intrachirp velocity variation such as can occur if the target isvibrating. By employing two, counter-chirping lasers, these velocityinduced range errors can be compensated. The resulting signals can thenbe processed to provide a much greater immunity to velocity errors.Rather than using up chirp and down chirp from a single laser todetermine the Doppler corrected range as in equations (5) and (7) above,the corrected range can be Doppler compensated by using just the upsweepof one laser and the corresponding down sweep of the other laser asshown in equation (9) below.

$\begin{matrix}{f_{1} = {\frac{f_{1\; {up}} + {\frac{\Delta \; f_{1}}{\Delta \; f_{2}}f_{2\; {down}}}}{2} = \frac{f_{1\; {down}} + {\frac{\Delta \; f_{1}}{\Delta \; f_{2}}f_{2\; {up}}}}{2}}} & (9)\end{matrix}$

This method not only greatly reduces Doppler errors but also effectivelydoubles the measurement rate reducing a single measurement time fromT_(mod) to one-half of T_(mod).

If the accuracy of range measurement is limited by the linearity of thefrequency modulation over the counting interval, then, if the target isone meter distant, a linearity of one part per thousand can provide 1 mmaccuracy. If real time variances from linearity can be detected andcompensated for, range measurement with single digit micron precisioncan be accomplished. FM lasers can be immune to ambient lightingconditions and changes in surface reflectivity because FM laser radarscan rely on beat frequency, which is not dependent upon signalamplitude, to calculate range. This can enable an FM coherent system tomake reliable measurements with a very small amount, for example, onepicowatt, of returned laser energy, or a nine order-of-magnitude dynamicrange of sensitivity.

The basis of coherent FMCW reflectometry is the heterodyne mixing of twosignals originating from the same linearly chirped source, one signalfollowing a local oscillator “LO” path, while the other reflects backfrom the target. Any time delays between the signals reflected back fromsites along the test path and the signal from the reference reflectiongive rise to beat frequencies in the mixed output. The values of thebeat frequencies are proportional to the time delays, while the sizes ofthe signals at the beat frequencies are proportional to thecorresponding reflection factors. A spectral analysis of this outputtherefore reveals the locations (relative to the reference path length)and strengths of any sites of reflection along the test path.

Referring now to FIG. 3, system 100 illustrates a configuration havingtwo laser sources in a counter chirp configuration that results in alaser radar optical configuration. The lasers are tuned with twodifferent tuning rates in a heterodyne mixing scheme to construct anembodiment of a dual chirp coherent laser radar that can be implementedon a planar lightwave circuit (PLC) and can be immune to environmentaleffects. In the present embodiment, the laser light can be, but is notlimited to being, generated by two polarization maintaining (PM) fiberpigtailed laser diodes LD1 13 and LD2 11. The light from laser LD1 13 isfrequency modulated at a rate that is a function of the tuning rate ofLD2 11. The light from each laser passes through fiber optic opticalisolator 12 to prevent back-reflected light from disrupting the tuningcharacteristics of the lasers. The light from each laser is combined in,for example, but not limited to, a 3×3 PM coupler 15 using five of thesix possible ports. The light can then travel down combined light pathbeam 17 (used interchangeably herein with fiber 17) and can betransported to the site of the measurement through gap 18 with geometricflexibility. At the end of fiber 17, the light emerges from combinedlight path beam 17, passes out of the fiber at the output 19 and ispartially reflected back into the fiber at the fiber end. The reflectedlight paths become local oscillator (LO) paths for each laser radar. Thelight that emerges from fiber 17, reflects off target 21, and returns tofiber 17 defines the signal path of the laser radar. Optionally, lensingsystem 27 can be used to focus the light in the measurement region ofinterest in order to maximize the amount of light returned to fiber 17.Lensing system 27 can be, for example, a fixed focus system or anadjustable focus system depending upon the optical depth of fieldneeded. In addition, after emerging from lensing system 27, the lightcan be directed to different parts of target 21 by means of an optionalscanning mirror (not shown) to provide 2D or 3D measurements. Uponemerging from output 19, the light from laser LD1 13 and LD2 11 islinearly polarized in one direction. Upon reflection from target 21, thelight from each laser is reinjected back into the fiber carryingcombined light path beam 17 in the same polarization axis. The light inboth the LO path and signal path travels back through coupler 15 tooptical detector 23 where the light from the two paths mix to form theRF signal that is proportional to the range difference between the twopaths. The RF signal can contain two frequencies that can correspond tothe tuning rate of each laser. Since the LO paths and the signal pathstravel in the fiber carrying combined light path beam 17 and fiber 24,the interference can be considered to be occurring at the interface 20that creates the LO signal, and thus, the path length difference betweenthe LO path and its respective signal path is equal to the distance tobe measured. Therefore, optical path changes in the fiber carryingcombined light path beam 17 and the fiber 24 carrying combined lightpath beam due to environmental effects such as temperature changes willhave no effect on the measured signal. The length of the fiber carryingcombined light path beam 17 from the coupler 15 to output 19 cantherefore be made arbitrarily long without degrading the measurement.This allows placement of a sensor head which can include, but is notlimited to including, output 19, lensing system 27, and scanningmechanisms, in areas of restricted volume while the remainder of system100, including, but not limited to, other optics, electronics, and powersupplies, can be located apart from the sensor head. In addition, an allfiber optic construction can provide for a ruggedized unit that canresist misalignment or degradation by airborne containments.

Continuing to refer to FIG. 3, for precision measurements referencestandard 41/42 can be included both for absolute ranging accuracy and tohelp linearize and monitor the chirp waveforms of the lasers. Referencestandard 41/42 can take the form of, for example, but not limited to, afiber optic interferometer in a Michaelson configuration. If the lengthof the fiber in the reference arm is calibrated, reference standard41/42 can serve as an absolute length standard for system 100 as well asprovide a signal useful in the linearization of first laser source 13and second laser source 11 waveforms. In a similar manner, referencedetector 29 can provide a signal useful in the linearization of bothlasers. Another useful use of reference standard 41/42 is to detect theexact tuning of the laser at a given instant which can be used todetermine the differential Doppler. Isolator 25 can isolate signal photodetector 23 from light from reference standard 41/42. The combination of2×2 coupler 41 and fibers 42 is a Michaelson reference arm whichoperates as follows: light enters 2×2 coupler 41 from isolator 25 and issplit between output fibers 42. Upon hitting the air/glass interface atthe ends of fibers 42, a portion of the light is reflected in each fiberwhich then travels back to 2×2 coupler 41 and on to reference detector29. The resulting mixed signals are associated with the tuning rate ofeach laser and with the length difference between fibers 42.

Continuing to refer primarily to FIG. 3, the method for determining ameasured distance between interface 20 of measuring device 20 A andobject 21 can include, but is not limited to including, the steps ofproducing first light beam 13A having a first frequency from first lasersource 13 and second light beam 11A having a second frequency fromsecond laser source 11, chirping up the first frequency at a first rateas the second frequency is chirped down at a second rate, chirping upthe second frequency at the second rate as the first frequency ischirped down at the first rate, combining first light beam 13A andsecond light beam 11, directing combined light beam path 17 towardobject 21, receiving reflected local oscillator (LO) path light beam 461(FIG. 11) associated with combined light beam path 17, receivingtarget-reflected signal path light beam 469 (FIG. 11) associated withcombined light beam path 17, and heterodyning LO path light beam 461(FIG. 11) and the target-reflected signal path light beam 469 (FIG. 11)to generate two different beat frequencies that are proportional to themeasured distance, the two beat frequencies being detected by singledetector 23.

Continuing to still further primarily refer to FIG. 3, the system 100for determining a measured distance between measuring device 20 A andobject 21 can include, but is not limited to including, first lasersource 13 for producing first light beam 13A having first waveform 32and first frequency, second laser source 11 for producing second lightbeam 11A having a second frequency, second light beam 11A having asecond waveform 36, wherein the first frequency is chirped up at a firstrate as the second frequency is chirped down at a second rate, and thesecond frequency is chirped up at the second rate as the first frequencyis chirped down at the first rate. System 100 can further includeoptical element 15 for combining first light beam 13A with second lightbeam 11A into combined light beam path 17. Optical element 15 can splita returning portion of combined light beam path 17 into third light beam24 (used interchangeably herein with fiber 24), and single detector 23for receiving third light beam 24 including two different beatfrequencies that are proportional to the measured distance.

Referring now primarily to FIG. 4, system 150 illustrates an alternateembodiment. This configuration can minimize back reflections and alsomake the circuit easier to implement on planer lightwave circuits. Itreplaces 3×2 coupler 15 (FIG. 3) by three 2×1 couplers 43, 45, and 47,and 2×2 coupler 41 (FIG. 3) by two 2×1 couplers 49 and 51. Note thatisolators 12 between first laser source 13/second laser source 11 and2×1 coupler 47 are not shown, but are part of system 150.

Referring now to FIGS. 5A and 11, system 200 illustrates a furtheralternate embodiment. This configuration is based on a polarizationdiplexing scheme which could increase the sensitivity of the system bydecreasing the amount of noise generated by light backscattering alongthe output fiber. In this geometry, the light from each laser iscombined in 3×3 polarization maintaining coupler 15. The light thentravels down the top fiber to polarization splitter 57. The light thattravels down fiber 61 is extinguished by isolator 63. The output lightfrom fiber 55 then exits fiber 55 and enters lensing system 27. In thiscase, the fiber end is angled such that no light is reflected back intofiber 55 from the interface 20. A ¼ wave plate 463 (FIG. 11) can convertthe optical polarization to circularly polarized light. The fiber endcan be angled such that no light is reflected back into fiber 55 fromthe air/glass interface. A partial reflector 465 (FIG. 11) can reflect asmall amount of the light back through ¼ wave plate 463 (FIG. 11) andinto the orthogonal polarization axis of output fiber 55. This lightserves as the local oscillator (LO). The light that emerges from lensingsystem 27 reflects off target 21, and returns to fiber 55 through ¼ waveplate 463 (FIG. 11) can define the signal path of the laser radar. Thislight is also in the orthogonal polarization axis. The light in both theLO and signal paths travels back through polarization splitter 57 and isdirected along second fiber 62 through isolator 63 to coupler 15 then tooptical detector 23 where the light from the two paths mix to form theRF signal that is proportional to the range difference between the twopaths, The RF signal contains two frequencies that correspond to thetuning rate of each laser. In this geometry, the backscattered light inthe output fiber is in an orthogonal polarization to the LO beam and itwill not coherently mix and thus will not produce a noise signal.

Referring now primarily to FIG. 5B, system 450 illustrates a furtheralternate embodiment. This configuration is based on a polarizationdiplexing scheme which could increase the sensitivity of the system bydecreasing the amount of noise generated by light backscattering alongthe output fiber. In this geometry, the light from each laser passesthrough isolator 12 and is combined in 2×2 polarization maintainingcoupler 41. The light then travels to polarization splitter 57. Fromthere the output light travels down fiber 17A then exits fiber output19. The fiber output end is angled to prevent light backreflecting backinto fiber 17A. Since all the components are polarization maintaining,the light emerges from fiber output 19 linearly polarized. The lightthen passes through ¼ wave plate 463 (FIG. 11) which converts the linearpolarized light to circularly polarized light, for example, right handcircularly polarized. Partial reflector 465 (FIG. 11) can reflect asmall amount of the light back through ¼ wave plate 463 (FIG. 11) andinto the orthogonal polarization axis of output fiber 19. This lightserves as the local oscillator (LO). The remainder of the light thatpasses through lensing system 27 emerges from lensing system 27,reflects off target 21, and returns to fiber 17A through ¼ wave plate463 can define the signal path of the laser radar. This light is also inthe orthogonal polarization axis. The light in both the LO and signalpath travels back through polarization splitter 57 and is directed alongto optical detector 23 where the light from the two paths mix to formthe RF signal that is proportional to the range difference between thetwo paths. The RF signal will contains two frequencies that correspondto the tuning rate of each laser. In this geometry, the backscatteredlight in output fiber 19 is in an orthogonal polarization to the LO beamand it will not coherently mix and thus will not produce a noise signal.

Referring primarily to FIG. 5B and FIG. 3, an alternative to thisgeometry is to use angled fiber/¼ wave plate/partial reflector lensinggeometry with the fiber geometry of FIG. 3. This geometry can preventthe noise due to the backscattered light as well as reduce the componentcount. A partial reflector can reflect a small amount of the light backthrough ¼ wave plate 463 (FIG. 11) and into the orthogonal polarizationaxis of output fiber 17. This light serves as the local oscillator. Thelight that emerges from lensing system 27, reflects off target 21, andreturns to fiber 17 through ¼ wave plate 463 (FIG. 11) defines thesignal path of the laser radar. This light is also in the orthogonalpolarization axis. The laser light is generated by PM fiber pigtailedlaser diodes LD1 13 and LD2 11. The light from laser LD1 13 is frequencymodulated a rate different than that of LD2 11. The light from eachlaser passes through fiber optic optical isolator 12 to preventback-reflected light from disrupting the tuning characteristics of thelasers. The light from each laser is combined in 3×3 polarizationmaintaining coupler 15. The light then travels down fiber 17 and can betransported to the site of the measurement with geometric flexibility.At the end of fiber 17, the light emerges from the fiber, passes throughthe ¼ wave plate 463 (FIG. 11) and is partly reflected by partialreflector 465 (FIG. 11). The reflected light paths become the localoscillator (LO) paths for each laser radar. The light that emerges fromfiber 17, reflects off target 21, and returns to fiber 17 defines thesignal path of the laser radar. Optionally, lensing system 27 can beused to focus the light in the measurement region of interest in orderto maximize the amount of light returned to optical fiber 17. Lensingsystem 27 can be, for example, a fixed focus system or an adjustablefocus system depending upon the optical depth of field needed. Furtheroptionally, after emerging from lensing system 27, the light can bedirected to different parts of target 21 by means of a scanning mirrorto provide 2D or 3D measurements.

Referring now to FIG. 6, there are applications of this technology thatcould benefit from having multiple output beams. System 250 is a furtheralternative embodiment based on the main concept of this disclosure. Insystem 250, the outputs of the two lasers are mixed together in 2×2coupler 65. Outputs 67 and 69 of coupler 65 feed into couplers 71 and 73which provide multiple output channels each. In this case, 1×3 couplers71 and 73 are used to provide for five output beams 75, 77, 79, 81, and85, and reference arm 83. Each output channel consists of a 1×2 couplerwhich directs the light to lensing systems 75A, 77A, 79A, 81A, and 83A,the fiber/glass interface which generates the LO and a detector fordetecting the two signal frequencies. Also shown is a Michaelsonreference arm composed of 2×2 coupler 91, two offset lengths of fiber 93and 95 and reference detector 97. Other reference arms could be used,for example, but not limited to, a Mach Zehnder configuration referencearm.

Referring now to FIG. 7, system 300 shows a geometry that uses delaylines to multiplex the various output signals onto one detector, thusreducing the cost and complexity of the radar. In this case, the lasersare combined in 2×2 coupler 101. Output 103 of coupler 101 feedsreference arm coupler 105. In this case, the reference arm is shown in,for example, but not limited to, a Michaelson configuration. Output 107is directed to 2×2 coupler 109 which in turn feeds the multiple outputfibers via 1×3 couplers 111 and 113. In this case, outputs 115 is usedto generate the LO via the reflection off the air/glass interface.Outputs 117, 119, 121, 123, and 125 are fibers with varying amounts offiber delays (D1, D2, D3, D4, and D5). The ends of these fibers areangled so that more LOs are not generated. Since there are varyingdelays, the signals that are generated by the light reflecting off thetarget returns a different frequency at detector 23. And since thelasers are being tuned at different rates, the signals from the lasersare also separated in frequency.

Referring now to FIG. 8, system 350 presents an alternate embodiment aconfiguration of the multi-beam concept. System 350 could minimize theback reflections and also make the circuit easier to implement on aplaner lightwave circuits. In system 350, the light is combined in 2×1coupler 201 and then split in 2×1 coupler 203, fiber 207 goes to thereference arm and fiber 205 goes to the signal arm. Fiber 205 isdirected to splitter 2×(n+1) 209, where n is the number of beams. Fiber211 carries the LO path and fibers 213A, 213B, . . . 213 n carry theoutput beams with different fiber length so that photo detector 23 cancarry all the signals which can be propagated depending on the differingdelays.

Referring now to FIG. 9, system 400 presents another alternatemulti-output embodiment. In system 400, the laser light is generated bytwo PM fiber pigtailed laser diodes 401 and 403. The light from laser401, designated the slow laser, is aligned with the slow axis of the PMfiber. The light from laser 403, designated the fast laser, is alignedwith the fast axis of the PM fiber. In system 400, the light from lasersL1 401 and L2 403, after passing through isolators 405 and 407respectively, are combined by a polarization splitter 409. Polarizationsplitter 409 can couple the light in the slow axis of fiber 411 and thelight in the fast axis of fiber 413 into the corresponding axes of fiber415. The light from the laser LD1 401 and LD2 403 are split into fibers417 and 419 by means of 1×2 fiber optic coupler 421 such that theirorthogonal polarizations are preserved. The light is then split intomultiple outputs by 1×3 couplers 423 and 425. Each output channel caninclude a 1×2 coupler which can direct the light to lensing systems 27,the fiber/glass interface which generates the LO, polarization splitters427 which separate the two signals generated by each laser, anddetectors 429 for detecting the signal frequency. The light from laserL1 401, which was in the slow axis, is reflected back into the slow axisand the light from laser L2 403, which was in the fast axis, isreflected back into the fast axis. These reflected light paths becomethe local oscillator (LO) paths for each laser radar. The light thatemerges from the fiber, reflects off the target and returns to the fiberdefines the signal path for each laser radar. Upon emerging from theoutput fiber, the light from laser L1 401 is linearly polarized in onedirection and the light from laser L2 403 is linearly polarized in adirection orthogonal to the laser L1 401 light. Upon reflection from atarget, the light from each laser is reinjected back into the fiber inits original axis. The laser L1 light in both the LO and signal pathtravels back through 1×2 couplers 431 and polarization splitters 427 tooptical detectors 428 where the light from the two paths mix to form theRF signal that is proportional to the range difference between the twopaths. In a similar manner, the laser L2 light from the LO and signalpaths travels to L2 detectors 429. Since the light from the two lasersare always in orthogonal polarization states, they do not interfere witheach other and the two resulting laser radar signals appear only attheir own detectors. Also shown is a Mach Zehnder reference arm composedof 1×2 couplers 435, offset lengths of fiber 437, polarization splitter439 to separate the two reference arm signals and two referencedetectors. A Michaelson configuration reference arm could also be used.The source lasers typically used in this application are diode laserswith output wavelengths centered around 1550 nm (near IR). Since thiswavelength is invisible to human vision, a second, visible laserfrequency can be added to the fiber optic circuit to aid the user byproviding a visible spot on the target at the same location as the IRmeasurement spot.

Referring now to FIG. 10, the light from a fiber pigtailed visible laserdiode 451 can be coupled into output fiber 453 by means of wavelengthdiplexing coupler 455. Coupler 455 can combine visible light fromvisible laser diode 451 with IR light from polarization splitter 457into output fiber 453.

Referring now to FIG. 11, the end of the fiber output 19 is angled. Thisis the interface between the air and the glass of the fiber. If this isnot angled, there is a reflection generated that returns along thefiber. This is how the LO path in FIG. 5A is generated. The lightemerges from fiber 19, goes through ¼ wave plate 463 and some of it isreflected off partial reflector 465. Some of the reflected light goesback through ¼ wave plate 463 and reenters fiber 19. This is the LOpath. The light is now in the orthogonal polarization due to the doublepass through ¼ wave plate 463. The ¼ wave plate 463 is at an angle toprevent spurious reflections from reentering the fiber. Optionally, side465 A of partial reflector 465 could also be angled or could beantireflection coated.

Therefore, the foregoing is considered as illustrative only of theprinciples of the present teachings. Further, since numerousmodifications and changes will readily occur to those skilled in theart, it is not desired to limit the present teachings to the exactconstruction and operation shown and described, and accordingly, allsuitable modifications and equivalents may be resorted to, fallingwithin the scope of the present teachings.

1. A method for determining a measured distance between an output (19)of a measuring device (20A) and an object (21) comprising the steps of:producing a first light beam (13A) having a first frequency from a firstlaser source (13) and a second light beam (11A) having a secondfrequency from a second laser source (11); chirping up the firstfrequency at a first rate as the second frequency is chirped down at asecond rate; chirping up the second frequency at the second rate as thefirst frequency is chirped down at the first rate; combining the firstlight beam (13A) and the second light beam (11A); directing the combinedlight beam path (17) toward the object (21); receiving a reflected localoscillator (LO) path light beam (461) associated with the combined lightbeam path (17); receiving a target-reflected signal path light beam(469) associated with the combined light beam path (17); andheterodyning the LO path light beam (461) and the target-reflectedsignal path light beam (469) to generate two different beat frequenciesthat are proportional to the measured distance, the two beat frequenciesbeing detected by a single detector (23).
 2. The method as in claim 1further comprising the step of: generating the first light beam (13A)and the second light beam (11A) by polarization maintaining fiberpigtailed laser diodes.
 3. The method as in claim 1 further comprisingthe step of: preventing back-reflected light from disrupting tuningcharacteristics of the first light beam (13A) and the second light beam(11A).
 4. The method as in claim 3 wherein said step of preventingcomprises the step of: optically isolating the combined light beam path(17).
 5. The method as in claim 1 further comprising the step of:focusing the combined light beam path (17) towards a measurement regionof the object (21).
 6. The method as in claim 5 wherein said step offocusing comprises: selecting a focusing means from a group consistingof a fixed focus system and an adjustable focus system.
 7. The method asin claim 1 further comprising the step of: producing an absolutedistance measurement through use of a reference standard (41/42).
 8. Themethod as in claim 7 wherein the reference standard (41/42) is a fiberoptic interferometer in a Michaelson configuration.
 9. The method as inclaim 1 further comprising the step of: preventing noise due tobackscattered light by use of an angled fiber (19A), a ¼ wave plate(463), and a partial reflector (465).
 10. A system for determining ameasured distance between a measuring device (20A) and an object (21)comprising: a first laser source (13) for producing a first light beam(13A) having a first waveform (32) and a first frequency; a second lasersource (11) for producing a second light beam (11A) having a secondfrequency, said second light beam (11A) having a second waveform (36),wherein said first frequency is chirped up at a first rate as saidsecond frequency is chirped down at a second rate, and said secondfrequency is chirped up at the second rate as said first frequency ischirped down at the first rate; an optical element (15) for combiningsaid first light beam (13A) with said second light beam (11A) into acombined light beam path (17), said optical element (15) splitting areturning portion of said combined light beam path (17) into a thirdlight beam (24); and a single detector (23) for receiving said thirdlight beam (24) including two different beat frequencies that areproportional to the measured distance.
 11. The system as in claim 10further comprising: polarization maintaining fiber pigtailed laserdiodes for generating the first light beam (13A) and the second lightbeam (11A).
 12. The system as in claim 10 further comprising: opticalisolators (12) for isolating the combined light beam path (17).
 13. Thesystem as in claim 10 further comprising: a lensing system (27) focusingthe combined light beam path (17) towards a measurement region of theobject (21).
 14. The system as in claim 13 further comprising: afocusing means in said lensing system (27) selected from a groupconsisting of a fixed focus system and an adjustable focus system. 15.The system as in claim 10 further comprising: a reference standard(41/42) producing an absolute distance measurement.
 16. The system as inclaim 15 wherein said reference standard (41/42) is a fiber opticinterferometer in a Michaelson configuration.
 17. The system as in claim10 further comprising: an angled fiber (19A); a ¼ wave plate (463); anda partial reflector (465), wherein a combination of said angled fiber(19A), said ¼ wave plate (463), and said partial reflector (465)prevents noise due to backscattered light.
 18. The system as in claim 10wherein said optical element (15) comprises: at least one polarizationmaintaining coupler (41).
 19. The system as in claim 10 furthercomprising: a polarization splitter (57) dividing the input from atleast one polarization maintaining coupler (41) into oppositelypolarized outputs.
 20. The system as in claim 10 further comprising: aplurality of couplers (71, 73) providing multiple output beams (75, 77,79, 81, 85).
 21. The system as in claim 10 further comprising: aplurality of output fibers (117, 119, 121, 123, 125) having fiber delays(D1, D2, D3, D4, D5).
 22. The system as in claim 13 further comprising:a visible laser frequency providing a visible spot on the object (21) inthe measurement region.
 23. The system as in claim 10 furthercomprising: a wavelength diplexing coupler (455) combining visible lightfrom a visible laser diode (451) and infrared light from a polarizationsplitter (457) into an output fiber (453).